1. Field of the Invention
The present invention relates to a method of manufacturing an ink jet head used within an ink jet printer. The present invention also relates to the ink jet head itself.
2. Description of the Related Art
A known technique for manufacturing an ink jet head is to join together a cavity unit and an actuator unit. The cavity unit has a plurality of nozzles and a plurality of pressure chambers. Each of the pressure chambers joins with a corresponding one of the nozzles. The actuator unit comprises a plurality of piezoelectric elements. When the cavity unit and the actuator unit are joined together, each piezoelectric element is located to face a corresponding one of the pressure chambers. Deformation of the piezoelectric elements applies pressure to ink filling the pressure chambers.
At the time of a printing operation, the piezoelectric elements are selected in accordance with the pattern of printing desired. Voltage is applied to the selected piezoelectric elements. The piezoelectric elements that have voltage applied thereto deform due to piezoelectric effects. When the piezoelectric element deforms, there is a contraction in capacity of its corresponding pressure chamber, pressure is thus applied to the ink filling the pressure chamber, and the ink is discharged from the nozzle connecting with the pressure chamber.
In order to obtain satisfactory printing, it is important to control the ejection speed of the ink being discharged from the nozzle such that this speed is constant. If the ejection speed is too fast or too slow, it is consequently not possible to obtain satisfactory printing.
It is known that there are various causes of fluctuation in the ejection speed of the ink. When the present inventors were researching the causes for such fluctuation, they learnt that large fluctuations were caused by: nozzle diameter, capacitance of the piezoelectric element in the vicinity of the pressure chamber that connects with the nozzle, and the voltage applied to the piezoelectric element. That is: the greater the nozzle diameter, the slower the ink ejection speed; the greater the capacitance of the piezoelectric element, the faster the ink ejection speed; and the greater the voltage applied to the piezoelectric element, the faster the ink ejection speed.
Since the nozzle diameter of the cavity unit is extremely small, it is difficult to process all the nozzles such that they have a uniform diameter.
Numerous nozzles are present in the cavity unit, and consequently there is variation in nozzle diameter even within the same cavity unit. The printer manufacturer produces the cavity units in quantity, and consequently there is also variation in nozzle diameter between one cavity unit and the next. In this latter case, the average nozzle diameter of the nozzles within the cavity unit varies from one cavity unit to the next.
Improved processing techniques have made it possible to reduce the degree of variation in nozzle diameter within the same cavity unit. By contrast, it is difficult to reduce the variation whereby the average nozzle diameter of the nozzles within one cavity unit varies the average nozzle diameter within other cavity units.
Further, the actuator unit is usually manufactured by making a plurality of folds in an extremely thin sheet. Since the piezoelectric elements within the actuator unit are formed from the same sheets, there is a small degree of variation in the capacitance of the piezoelectric elements within the same actuator unit. By contrast, it is difficult to reduce the variation whereby the average capacitance of the piezoelectric elements within one actuator unit varies the average capacitance in other actuator units. It is difficult to reliably control the thickness of the extremely thin sheets. Therefore, it is assumed that the variation in capacitance is caused by the variation in the thickness of the sheets of each actuator unit.
As described above, there is a degree of variation that cannot be tolerated between the average nozzle diameter of nozzles within one cavity unit and that in other cavity units. Similarly, there is a degree of variation that cannot be tolerated between the average capacitance of the piezoelectric elements within one actuator unit and that in other actuator units.
Due to this variation between units, there is a variation that cannot be tolerated in the ejection speed of the ink discharged from differing ink jet heads each made by joining together a cavity unit and an actuator unit. As described earlier, each ink jet head comprises a plurality of nozzles. Improved processing techniques have made it possible to reduce the degree of variation in the ink ejection speed between the nozzles in the same ink jet head. However, it is extremely difficult to reduce the variation of the average ink ejection speed between ink jet heads.
The present applicants have succeeded in reducing the variation of the average ink ejection speed between ink jet heads. This was done by adopting the following technique (Japanese Patent Application Publication No. 2003-11376; U.S. Pat. No. 6,796,631). The present applicants disclosed a relational expression that uses the average nozzle diameter of the nozzles within the cavity unit and the average capacitance of the piezoelectric elements within the actuator unit. This relational expression is used to calculate the voltage required to realize a determined average ink ejection speed when the cavity unit and the actuator unit have been joined together. When this relational expression is used, it is possible to determine the voltage to be applied to the ink jet head that has been formed by joining together these units. This is achieved by measuring the average nozzle diameter of the nozzles within the cavity unit, and the average capacitance of the piezoelectric elements within the actuator unit. When the voltage that has been determined in this manner is applied, the average ink ejection speed of the nozzles in the ink jet head is adjusted so as to be constant. Below, for the sake of simplicity, the average ink ejection speed of the nozzles within the ink jet head will be referred to as average ejection speed. The average nozzle diameter of the nozzles within the ink jet head will be referred to as average nozzle diameter. The average capacitance of the piezoelectric elements within the ink jet head will be referred to as average capacitance.